The function of the heart is to impart energy to the arterial blood which is then dissipated in the circulation. The energy transfer by the heart is associated with liberation of heat, i.e., a certain metabolic cost to the heart. The ratio of useful energy, i.e., the external work, over the metabolic cost, is the mechanical efficiency. In this project, efficiency is measured as work/(work plus heat) in chronically implanted dogs. Variability and possible control of left ventricular efficiency will be studied under normal and diseased states of the heart or circulation. It is predictable that efficiency should vary with the inotropic state of the heart, with changes in preload or afterload and with metabolic changes of the myocardium, e.g., diabetes. It is predictable that efficiency should vary with the inotropic state of the heart, with changes in preload or afterload and with metabolic changes of the myocardium, e.g., diabetes. It is also of interest to study the effects of common therapeutic interventions and drugs on efficiency. In the studies here proposed, left ventricular heat dissipation rate will be determined from coronary blood flow using isotope labeled microspheres and coronary a.v. temperature difference. Not all of the left ventricular heat is dissipated via the coronary blood. However, the coronary fraction can be determined from the coronary fraction of a known quantity of heat which is injected into the left coronary artery.